The Blair Witch Project from 1999 had a budget of barely $22,500 and managed to earn almost $250 million.
Has there ever been any other Science Fiction/Fantasy movie that had a better (or similar) income/budget ratio?
According to IMDb's trivia page and the Guinness Book of Records, no:
This film was in the Guinness Book Of World Records for "Top Budget:Box Office Ratio" (for a mainstream feature film). The film cost $22,000 to make and made back $240.5 million, a ratio of $1 spent for every $10,931 made.
According to this site, yes: by their figures, Paranormal Activity (another found-footage supernatural horror film) comes out on top with a budget of $15,000 and a revenue of $193 million, compared to The Blair Witch Project's budget of $600,000 and revenue of $248 million.
I tend to believe IMDb and Guinness. $600,000 seems like a very high budget for a cast of three pretty much unknown actors running around in the middle of a wood for a few days with cheap cameras.
Paranormal Activity supposedly had a budget of just $15,000 dollars and was filmed with two actors inside the Director's own home using rented HD cameras and a sound system "borrowed" from a local film studio by one of the crew. The film took over $193m at US and overseas box offices, making it the single most profitable film of all time at a ratio of approx $12,000 made for every dollar spent.
That being said, both The Blair Witch Project and Paranormal Activity benefited from enormous (multi-million dollar) after-market advertising campaigns and being picked up (and professionally re-edited) before receiving general release so it's really, really difficult to work out how much was actually spent on each film, let alone the post-editing, foley and other work that went into making them fit for movie theatres.
... in 2008 it [Paranormal Activity] came to the notice of DreamWorks. They felt it would be a good film to remake, and offered a hesitant Peli $350,000. However, the film's producer convinced DreamWorks to release Peli's version after Steven Spielberg became one of its most vocal champions. It was his suggestion that the now-famous end scene be added, and with this the film was eventually released on a limited theatrical basis. Paramount utilized social media to create demand, and Senior Vice-President of Interactive Marketing Amy Powell used the online 'Demand It!' Service where audiences could vote for certain films to play in their region, granting Paranormal Activity the "grassroots, do-it-yourself support ... from the bottom up" that she felt it required.'
While the $11,000 it cost to make the film is a more famous statistic in the original movie's now legendary ascension than the $2 million it cost to promote it, even with this figure added it was still a remarkable financial success: As early as January 2010 it had cleared over $90 million. - Found Footage Horror Films: Fear and the Appearance of Reality